1. Field of the Invention
The present invention relates to a damper, and more particularly to a valve structure for generating damping forces as a piston of a damper moves back and forth.
2. Description of the Prior Art
FIG. 14 of the accompanying drawings illustrates a conventional hydraulic damper. As shown in FIG. 14, the conventional hydraulic damper has a cylinder 101 and a piston rod 102 extending axially therein. A piston 103 is mounted on a distal end of the piston rod 102 in the cylinder 101 in sliding contact with an inner circumferential surface of the cylinder 101. The piston 103 divides the interior space of the cylinder 101 into two oil chambers S1 and S2, one on each side of the piston 103. The piston 103 has a compression oil passage 104 and an expansion oil passage 105 which are defined therein. A compression valve 106 for opening and closing the compression oil passage 104 and an expansion valve 107 for opening and closing the expansion oil passage 105 are disposed one on each side of the piston 103. The compression valve 106 has a slit 109a defined therein. The expansion valve 107 comprises a plurality of plate valves 111, 112, 113 positioned adjacent to a valve seat 110 of the piston 103 remote from the compression valve 106.
Each of the plate valves 111, 112, 113 is of a circular shape, and the valve seat 110 for bearing these plate valves 111, 112, 113 is of a circular (annular) shape as shown in FIG. 15(a) of the accompanying drawings. The valve structure of the conventional hydraulic damper has such damping force characteristics in an expansion stroke that, as indicated by the curve III in FIG. 6, when the speed of travel of the piston 103 is low, working oil in the oil chamber S1 flows through the slit 109a of the compression valve 109 and the compression oil passage 104 into the oil chamber S2, producing a relatively steep damping force characteristic curve, and when the piston 103 travels at a medium speed, working oil flowing into the expansion oil passage 105 forces the expansion valve 107 to open, producing a relatively gradual damping force characteristic curve because of the circular (annular) valve seat 110. Therefore, the circular (annular) valve seat 110 fails to provide linear damping force characteristics due to an inflection between the low- and medium-speed ranges.
Another hydraulic damper employs a noncircular (nonannular) valve seat as shown in FIG. 15(b) of the accompanying drawings for opening the valve gradually when the piston speed is low, so that substantially linear damping force characteristics are provided throughout the low- and medium-speed ranges. However, the noncircular valve seat causes the valve to open to a large extent, resulting in relatively steep damping force characteristics in a higher speed range, as shown in FIG. 6.
Japanese laid-open patent publication No. 60-99341 discloses a hydraulic damper which employs a circular valve seat and an expansion valve similar to the expansion valve 107 shown in FIG. 14. The expansion valve includes an auxiliary plate valve 111, an intermediate plate valve or sheet 112, and a main plate valve 113, the intermediate plate valve 112 having an eccentric shape as shown in FIG. 15(c) of the accompanying drawings. The expansion valve flexes gradually in its fully circumferential region after portions of the auxiliary and main plate valves 111, 113 have started flexing, thereby eliminating any inflection from low- and medium-speed ranges.
While the valve structure disclosed in the above publication provides the same damping force characteristics in the medium-speed range as those of the valve structure with the circular valve seat, it is difficult to grasp the correlation between the configuration of the intermediate plate valve and the generated damping forces.